Rotary encoder having a plurality of beams emitted by a diffraction grating

Claim 1

A rotary encoder comprising: a rotary scale having a diffraction grating formed along a rotational direction thereof and a predetermined thickness distribution a; irradiating means for irradiating a first position of said rotary scale with a first radiation beam and a second position of said rotary scale, different from the first position, with a second radiation beam, and for generating first and second diffraction beams; and means for receiving an interference beam formed by superposing the first diffraction beam and the second diffraction beam emitted from said rotary scale with each other, and for converting the interference beam to a signal to measure the rotational state of said rotary scale in accordance with the signal with predetermined accuracy, wherein the thickness distribution a of said rotary scale satisfies the equation: a=tan.sup.-1 (Δk/l)≦tan.sup.-1 [{π(m-n)λ/360n.sub.0 ·p}·θ.sub.k ] for n 0 including 1, where n 0 is a refractive index of said scale, p is a pitch of said diffraction grating, λ is a wavelength of the first and second radiation beams, m and n are orders of the diffracted beams of the specific orders, l is a distance between the different positions, Δ k is an optical path difference generated when the first and second radiation beams pass through the different positions once, and θ k is an allowable value of an angular error of said encoder.